1. Field of the Invention
This invention relates to a catadioptric lens system which forms an image by means of both the refraction and the reflection of the light rays.
2. Description of the Prior Art
There is known in the art that the catadioptric lens system is adapted for ultratelephoto lens systems since the total length of the system is remarkably reduced and the spectra of the second degree are decrease by the adoption of the catadioptric lens system.
Various catadioptric lens systems have been suggested such as those disclosed in U.S. Pat. No. 3,507,556; U.S. Pat. No. 3,632,190; and Japanese Patent Applications laid open to the public inspection under Nos. 52-155549 and 53-76830.
In the catadioptric lens sytem, however, it is difficult to increase the field angle of the lens system, and in fact, field angles less than 5 degrees are of known catadioptric lens systems practically designed to use in 35mm SLR camera. This difficulty in increasing the field angle of catadioptric lens system is caused by its characteristic light path which reciprocates between the main-mirror and the sub-mirror. Although it is required to avoid the possible intersection of the light pencils directed to or reflected on the main-mirror with the light pencils traveling from the sub-mirror to the image surface, it becomes extremely difficult to meet this requirement as the field angle is increased. In other words, an off-axial light pencil reflected on the sub-mirror is caused to pass through the main-mirror at a portion considerably off the optical axis of the system when the field angle is to be increased, which means that the main-mirror should have a sufficiently large inner effective diameter to allow the passage of such an off-axial light pencil. This leads to an inevitable increase in the outer effective diameter of the main-mirror if a sufficient illumination by the off-axial rays is desired to be maintained. Thus, an undesirably thick lens system would result from the increase of the field angle. Further, the excessively large inner effective diameter of the main-mirror results in a deterioration of the lens performance caused by the diffraction of light.